1. Field of the Invention:
The present invention relates to a guide-rail brake and a method for controlling a guide-rail brake.
2. Background of the Invention
A guide-rail brake of a prong type that grasps the guide rail, in which compression of the braking surfaces exerted on the guide rail is achieved by means of a spring force, is very generally used a brake device. This type of brake device is often a safety apparatus to stop an upward or downward moving elevator, if the speed has become to high or for some other reason. In practice placement the brake on the elevator car, especially adding one to an existing elevator, is problematic in terms of space usage, because the brake must be quite large owing to the large forces exerted on the brake. A prong-type brake can also be the operating brake of an elevator. A prong-type operating brake has been used in e.g. linear motor elevators. Recently these type of brakes have been presented in publications U.S. Pat. No. 5,518,087 and EP 0 488 809 A2, among others. These types of brakes contain braking surfaces or brake pads on the tip of the prong, which are located at a distance from the guide rail. A spring is disposed in the prong, and the loading on the prong achieved by the spring endeavors to press the brake pads or braking surfaces against the guide rail. Preventing this endeavor is a holding device, usually an electromagnet. This or a separate electromagnet is used to open the brake. Publication FI 970390 also presents a good example of an effective guide-rail brake.
In prior-art guide-rail brakes the fixed hinging of the brake prongs causes a need to manufacture a specific brake prong for each brake size. If this is not done, a misalignment between the guide rail and the braking surfaces arises when using a guide-rail brake with a guide rail of a different thickness than that for which the guide-rail brake is intended. Owing to the misalignment the contact surface between the braking surfaces and the guide rail is reduced, so the surface pressure on the contact surface is great. Due to the great local surface pressure, the braking surfaces wear unevenly and quickly. Also wear of the guide rail increases compared to the case where the braking surfaces and guide rail surfaces are aligned. Another result of the misalignment is that uneven surface pressure is exerted on the guide rail, in which case the guide rail wears unnecessarily. The dependency of the brake on the guide rail size reduces manufacturing batch sizes and increases warehousing costs and other costs.
In prior-art brakes there is a fairly large clearance between the guide rail and the braking surface to ensure that the braking surface does not touch the guide rail when the brake is open. A result of the large clearance between the guide rail and the braking surface is a need for a large stroke length of the pulling of the magnet to open the brake, which in turn creates a need to increase the size of the magnet. The long stroke required by the large clearance also causes noise problems, because closing of the brake occurs by means of a spring. During the long stroke greater energy from the spring is exerted on the movement of the prong than would be in a short stroke. The placement of prior-art brakes is awkward, because when disposed below or on top of the elevator car they increase the overall height of the elevator car. Especially in the modernization of old elevators increasing the height of the elevator car may result in a need to lengthen the elevator shaft to ensure adequate headroom. Lengthening the elevator shaft has unfavorable cost repercussions however. Likewise substantial modification of the structure of the existing elevator car incurs considerable additional costs. Furthermore, a problem with prior-art guide-rail brakes is that their operating delay is too great, so they are only suitable for use in preventing overspeed upwards and downwards. At its fastest the delay may be e.g. approx. 500 ms and in that case it is possible that brakes according to prior art do not release at all owing to remanence and thus the brake device does not operate at all. The delay is often too long and the distribution of the delays of the different braking devices of the elevator is large. When the load is over 1000 kg and two guide-rail brakes are used, the distribution in delays means these devices operate at different times. A further problem is that guide-rail brakes according to prior art do not necessarily give full braking force immediately they are released, because residual magnetism causes a counterforce.